Post on 02-Jan-2016
Harry Roberts Harry Roberts Coastal Studies InstituteCoastal Studies Institute
School of the Coast and EnvironmentSchool of the Coast and EnvironmentLouisiana State UniversityLouisiana State University
River Diversions:River Diversions:Geological Boundary Conditions Geological Boundary Conditions
and Other Considerationsand Other Considerations
• Crustal Downwarping
• Compaction-Dewatering of Young Deposits
• Faulting
• Decreasing Sediment Supply-Increasing Accommodation
Key Geologic Factors in Land Loss
River Diversions: River Diversions: Best Hope for Off-Setting Best Hope for Off-Setting
Coastal Land LossCoastal Land Loss
Sediment Storage in Sediment Storage in the System Before the System Before
Human Intervention Human Intervention
VicksburgVicksburg
Baton RougeBaton Rouge
New MadridNew Madrid
MemphisMemphis
New OrleansNew Orleans
MississippiMississippiincised valleyincised valley
The Lower The Lower Mississippi River Mississippi River
and Deltaand Delta
• Glacial-period Glacial-period braided streams braided streams within incised within incised valleyvalley
• Holocene valley Holocene valley filling and delta filling and delta constructionconstruction
• Valley fill reflects Valley fill reflects interactions interactions between climate between climate and sea-level and sea-level changechange
Post-Glacial Alluvium and Deltaic Sediments = ~60.500 km2
Post-Glacial Alluvium and Deltaic Sediments = ~60.500 km2
Longitudinal Profile of the Longitudinal Profile of the Lower Mississippi Valley and DeltaLower Mississippi Valley and Delta
Tracing Late Pleistocene Braided Streams into the Tracing Late Pleistocene Braided Streams into the Subsurface Using Base of Backswamp DepositsSubsurface Using Base of Backswamp Deposits
Based on 325 USACE boreholes from Blum et al. (2008)
braided stream deposits
Lower Mississippi Valley and DeltaMagnitude of Post-Glacial Deposition
Total storage = 1860-2300 km3 or 2790-3450 BT of sedimentStorage rate = ~230-290 MT/yr over 12,000 yr post-glacial period
sediment isopachs adapted from Kulp (2000)sediment isopachs adapted from Kulp (2000)
Balize Delta > 100 m Thick
Mississippi River Discharges and Mississippi River Discharges and Sediment StorageSediment Storage
Pre-DamPre-DamSediment Load*: ~400 – 500 MT/yrSediment Load*: ~400 – 500 MT/yrMean Sediment Storage**: Mean Sediment Storage**:
230 – 290 MT/yr 230 – 290 MT/yr
1976-20061976-2006Mississippi & Atchafalaya Sediment Mississippi & Atchafalaya Sediment
Load: ~205 MT/yrLoad: ~205 MT/yr
*Suspended Sediment Load Meade et al. (1990); *Suspended Sediment Load Meade et al. (1990); Kesel et al. (1992) **Avg. over 12 kyrsKesel et al. (1992) **Avg. over 12 kyrs
Lower Mississippi River Sediment LoadLower Mississippi River Sediment LoadPre- and Post-Dam LoadsPre- and Post-Dam Loads
Data courtesy of USGS Baton Rouge
Modern post-dam sediment loads are ~65% of the long-Modern post-dam sediment loads are ~65% of the long-term mean storage component alone term mean storage component alone
Data courtesy of USGS Baton Rouge
230-290 MT/yr
Projections of Projections of Sea-Level Rise and Sea-Level Rise and
SubsidenceSubsidence
Increasing AccommodationIncreasing Accommodation
Global Sea-Level RiseGlobal Sea-Level RiseSea-Level Change Data and ProjectionsSea-Level Change Data and Projections
Satelliteobservations
Tide-gaugeobservations
from IPCC 2007
~1mm/yr ~1.7 – 2.0 mm/yr
~3.0 mm/yr
Baton RougeBaton Rouge
LafayetteLafayette
Projected future land loss of 10,500-13,500 km2
The Louisiana Coast in 2100?
New accommodation ~12-16 km3
(Requires ~18-24 BT to fill accommodation)
New New OrleansOrleans
New New OrleansOrleans
Baton RougeBaton Rouge
LafayetteLafayette
Projected future land loss of 10,500-13,500 km2
1400-1800 km2
with 50% ofsediment
load
The Louisiana Coast in 2100?
Mass balance Mass balance considerations considerations present tough choices present tough choices for diversion for diversion scenariosscenarios
A Goal of River Diversions:A Goal of River Diversions:Maximize Retention of Sediment Maximize Retention of Sediment
Within the Coastal PlainWithin the Coastal Plain
Fundamental Questions: River Diversions
• How are the sediments partitioned within the coastal-shelf system?
• What is the sediment retention capability in the delta and adjacent marshland?
• What are the important processes linked to sediment transport to the delta-marsh-offshore?
• How are the sediments partitioned within the coastal-shelf system?
• What is the sediment retention capability in the delta and adjacent marshland?
• What are the important processes linked to sediment transport to the delta-marsh-offshore?
Important Positive Impacts of Important Positive Impacts of Hurricanes, Tropical Storms Hurricanes, Tropical Storms
and Cold Frontsand Cold Fronts
Sediment Accretion
Inland Marsh Site M1Inland Marsh Site M1 Wax Lake Delta Site 9Wax Lake Delta Site 9
4 cm / 6 months4 cm / 6 months 1.5 cm / 5 months1.5 cm / 5 months
Impact of Cold Fronts:Impact of Cold Fronts:Less Energetic but More Less Energetic but More
Frequent than Tropical StormsFrequent than Tropical Storms
Cold Front Modulation of Sediment Transport
• Prefrontal:
– Onshore Winds
– Water Level Set-Up
– Onshore Sediment Transport
• Prefrontal:
– Onshore Winds
– Water Level Set-Up
– Onshore Sediment Transport
• Postfrontal:
– Offshore Winds
– Water Level Set-Down
– Offshore Sediment Transport
• Postfrontal:
– Offshore Winds
– Water Level Set-Down
– Offshore Sediment Transport
What Are Implications of What Are Implications of River Diversion Locations?River Diversion Locations?
Prefrontal Conditions
RESULT: Marshward Transport of Sediment (20-30 times/year)RESULT: Marshward Transport of Sediment (20-30 times/year)
Postfrontal Conditions
RESULT: Sediment Deposition, Marsh Substrate Accretion, Nutrient LoadingRESULT: Sediment Deposition, Marsh Substrate Accretion, Nutrient Loading
Project Results Suggest:
Placement of River Diversions in Landward
Parts of Interdistributary Basins Will
Maximize Sediment Retention Through
Landward Suspended Sediment Transport
by Cold-Front-Related Processes.
Placement of River Diversions in Landward
Parts of Interdistributary Basins Will
Maximize Sediment Retention Through
Landward Suspended Sediment Transport
by Cold-Front-Related Processes.